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Why use a TXV

The reason that a TXV is better that a piston is beocuse your TXV is suppost to maintain the superheat at any condition. If your Evaporator is not oversized to much you will not see a big advantage to using a TXV The way to be sure is to chare your system on a 95 day using a piston to 5-7 deg at compressor. When your ambient starts to drop let say to 80 deg check your superheat and if is not above 20 deg @ comp then you will not regain the cost of installing a TXV. Unless it is a non bleed (then you might need hard start componets). WHere you get your efficency gain is on shut down of the system. See most all equipment comes equiped with time delay for the blower. and te reson for this is becouse when a system starts up the evaporator is hot so you start up the condenser 7-10 sec. before the blower comes on so that when you do the air is already cold. THe same applies for the shut down when the thermostat satisfies then the condenser turns off 1 min. before the blower and this is becouse the evaporator is cold and there is not reson to leave the cold air in the duct work. SO as soon as the condenser shuts down then the sytem pressure starts to equilize and the high side (hot) starts to equilize to the low side(cold) and the non bleed will stop or slow down the amount of time it takes for your pressures to equilize and for that 1 min that your blower is running your evaporator will remain colder for a longer duration. so you will pick up a better effiececy. Will you regain the inital cost of instaling the TXV and possiably Hard start.? THat depends in the price.

The majority of work done by an evaporator is through latent heat of vaporization.
Not by superheat.
Evaporator superheat is insurance for your compressor. Period.
Extra superheat costs money (electricity), just like extra car insurance costs money.
As long as you have some superheat, you are not allowing liquid refrigerant to reach the compressor.
As long as you don't have excessive superheat, the refrigerant still has enough capacity to absorb heat from the compressor and keep it cool.
The TXV (TEV) allows the evaporator, to operate with minimal superheat, yet never risk floodback to the compressor. Some large systems operate with VERY minimal superheat (1/2 degree), but very closely controlled.

Using a cheaper, simpler, fixed-bore device is acceptable if the load and outdoor ambient are fairly constant. Most all domestic refrigerators and window units are fixed bore.

Because most consumers purchase the cheapest thing that will work, and many installers prefer fixed-bore, then the majority of home central air conditioners have fixed-bore metering devices. Problems arise when the air conditioner is operated in less-than-perfect conditions.

Since heat pumps must run year-round, TXV is the best choice, especially on the outdoor coil.
Some systems use fixed bore on the indoor coil, and TXV for outdoor. One of the reasons is the close proximity of the coil to the compressor.
There is no long suction line that can absorb additional sensible heat into the refrigerant (superheat) before it reaches the compressor.
Accumulators are installed as protection for the compressor, as well, for the same reason.

A matched indoor coil with a TXV, will usually outperform an oversized indoor coil with a fixed bore device, if ALL factors are considered.

The coil, fed from a TXV, will always run fully and properly loaded, and efficiently.
A coil, fed with fixed-bore, will run efficiently only some of the time, hopefully most.

If nothing else, a TXV will provide superheat control, in the event somebody "forgets" to change the filter, or the fan fails.

Originally posted by wendel he said with the coil being larger than the condenser a piston will be fine since there will never be too much R-22 to the coil. A TXV would never close, he said. Is he right??

You need to find yourself another A/C Company.

I am impressed that you can make that determination based on one sentence.

Originally posted by Bulldog I am impressed that you can make that determination based on one sentence.

Don't take much of a rocket scientist to figure out a TXV is better all around metering device and that is what his A/C guy should have been telling him, go with a TXV if he was interested in the best for his customer, Yea sometimes I even impress myself just wished it would have been something to really debate that question was pretty much a given for any good service tech.

__________________________________________________ _______________________Experience is a hard teacher because she gives the test first, the lesson afterwards". -Vernon Law-